Most combustion gas trains are required by national and/or local codes to use two safety shut-off valves in series on the main line to the burner. Depending upon the capacity rating, two safety shut-off valves in series may be required on the pilot line as well. Double block valves have been designed to meet this requirement by providing two independent sealing members in series in a single valve body. By eliminating the need for two separate valve bodies, the double block valve reduces the length of the fuel train and the requisite installation costs. Additionally, the double block valve eliminates several piping connections, which incrementally reduces the probability of external leaks. In some prior art double block valves, the two sealing members of the double block valve have impeded the fluid flow through the valve body and thus reduced the valves effectiveness. Existing double block valves may also have a substantial captured volume between the sealing members, which holds a large amount of gas or liquid when the sealing members are sealed.
Some installations on fuel trains use a double block valve with a vent system that places a normally open valve between the two sealing members. Any leakage from the upstream sealing member is vented to the atmosphere. Designing the venting system may prove to be expensive and may further complicate the proper sealing of the two sealing members in the double block valve. Furthermore, the venting system may expel the volatile organic compounds into the atmosphere, which is environmentally undesirable.
The same codes that require two safety shut-off valves on the burner line also mandate that the safety shut-off valves be tested on a regular basis to insure the integrity of the internal seals or blocks. The testing is usually performed manually and involves removing plugs from test ports on the valve body. For example, if a gaseous fuel is being valved, a combination of manual test cocks, rubber tubing, and a container filled with solution may be used to check for leakage of the sealing members in the double block valve. The appearance of gas bubbles in the solution identifies leakage. The determination of an acceptable leakage rate is often based upon the operator's judgement of the size and number of bubbles present.
The present invention is directed to an improved double block valve and a proving system and method, which may be used with first and second safety shut-off valves or with the improved double block valve.